Planting tomatoes properly can be a bit of a mystery.
First dig the hole about 10-12 inches deep. Add a handful of compost and a little fertilizer to the hole (I use a teaspoon of Pro-Gro, or Compost Plus). Stir it in. Fill the hole with water and let it thoroughly soak in.
Remove the tomato from its pot and gently loosen the root ball by squeezing it and wiggling your fingers along the bottom most roots. Gently snip off the bottom-most leaves and stems so that you can plant the tomato a little deeper. This will encourage the plant to shoot out more roots which will give it better structural support to bear the weight of future fruit. I also pull off any blossoms and fruits at this point to encourage the plants energy to go toward developing roots for its first week in the ground.
Place the plant in the hole and stabilize it with a little soil, then fill the hole with water again. Insert a strong stake securely into the hole a few inches from the root ball. Be sure that no leaves are touching the soil; if there are, snip those off too since tomato diseases are often soil-borne. Be sure to leave at least two full sets of leaves on the plant so that photosynthesis can still take place. When the water drains out, fill the rest of the hole with soil, pat it down gently, and water it again.
Cover the wet soil with mulch leaving a circle of exposed soil around the stem. If the mulch gets too close to the plant, it will promote rot and slugs. As the tomato grows, use a strip of cloth, string, or garden wire to tie the main stem of the plant to the stake.